Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive Income, Aggregated by Length of Time Investments (Detail) (USD $) In Thousands, unless otherwise specified	Aug. 31, 2013	May 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 48,861	$ 36,582
Unrealized losses with a loss position for less than 12 months, fair value	46,046	36,327
Unrealized losses with a loss position for more than 12 months, fair value	2,815	255
Total investments with unrealized losses, gross unrealized losses	(2,365)	(1,034)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(2,277)	(956)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (88)	$ (78)